Copilot Chat: All previous session history silently lost after upgrade (.json → .jsonl format migration not performed)

Does this issue occur when all extensions are disabled?: Yes/No

- VS Code Version: 1.127.0 (Universal)
- OS Version: macOS 26.5.1 (25F80)

Steps to Reproduce:

## Bug Description

After upgrading VS Code, all previous Copilot Chat session history is completely
gone from the Chat History panel. No warning, no migration, no recovery option.

## Root Cause (Observed)

The Copilot Chat extension changed its chat session storage format from `.json` to
`.jsonl` between versions. Sessions in the old `.json` format still exist on disk at:

~/Library/Application Support/Code/User/workspaceStorage//chatSessions/.json

...but the new extension silently ignores them. They are never read, migrated, or
displayed in the UI.

Related: #307761 (chat history not loading from .jsonl files — different root cause, closed as not planned)

## Steps to Reproduce

1. Use VS Code Copilot Chat for multiple sessions across several workspaces
2. Upgrade VS Code to a version that changed the session format to `.jsonl`
3. Open VS Code and check the Chat History panel
4. All prior sessions are gone

## Expected Behavior

One of:
- The extension **migrates** existing `.json` session files to `.jsonl` on first run after upgrade
- The extension **reads both formats** for backward compatibility
- Users are shown a **warning** before upgrading and offered an export/backup option

## Actual Behavior

Previous chat sessions vanish silently. No warning, no migration, no recovery UI.
Months of conversation context (debugging sessions, architectural decisions, code
explanations) is permanently lost.

## Impact

- Sessions represent significant complex work context that cannot easily be reconstructed
- The change was made without any migration path, user notice, or changelog callout

## Environment

- OS: macOS 26.5.1 (25F80)
- Old format: `.json` (files still present on disk, confirmed)
- New format: `.jsonl`

## Requested Fix

Add a one-time startup migration that converts `.json` chat sessions to `.jsonl`,
or at minimum add a "Restore legacy sessions" command to the Command Palette.